Vulnerability is not Sacha Baron Cohen's strong suit, but he is at a vulnerable point right now. Too famous to pull off the gonzo-prank format he pioneered with Borat and Brüno, he is forced to make the running jump to proper, 100% dramatised features. And he won't be doing it, like on Ali G Indahouse, as a primarily British concern, but in front of the eyes of the world as a global star on the verge of the A-list. You never could accuse Baron Cohen of lacking boldness in a nervy moment, though, and his new film The Dictator seems calculated to draw comparisons with comedy's highest and holiest. With that title, how can it not bring to mind Charlie Chaplin?

The plot, or what little we know about it, has strong echoes of The Great Dictator: in an adaptation of a novel by Saddam Hussein, Baron Cohen plays both the Middle Eastern autocrat and the goatherder who replaces him, straddling uber- and untermensch in the same spirit as Chaplin in the 1940 classic. (As the older film's preamble has it: "Any resemblance between Hynkel the dictator and the Jewish barber is purely coincidental.") The visual design in the first official shot of tyrannical Baron Cohen is fantastic: part-Gaddafi, part-Assyrian god-king, all nasty, post-colonial crackpot pomp - with even a hint of Lynchian freakiness.

I don't know how I didn't notice Baron Cohen's Chaplin worship before. It's obvious: Borat is a clown Everyman in the tradition of The Tramp and even looks like his Eurasian brother, with his 'tache, bad suit and awkward gait. Both characters shuffle along on similar lines: the naif, adrift in a fast-expanding world, but buoyed up by an unshakable optimism. The Tramp navigated the snakes and ladders of early 20th-century American life; Borat is doing the same in the globalised maelstrom nearly 100 years later.

But Baron Cohen has picked the toughest of acts to follow, so let's hope The Dictator is very good indeed. We'll never know how Chaplin would have fared in a real-life interview with the al-Aqsa Martyrs' Brigade, but in a controlled environment he was a master physical actor, every tic and inflection impeccably micromanaged. Baron Cohen has had clown training, too, at Ecole Philippe Gaulier near Paris. But his track record in conventional films – the Nascar nemesis Jean Girard in Talladega Nights and the Demon Barber's Italian rival, Pirelli, in Sweeney Todd – is less impressive. Both performances were 'eavy on ze outrageous accent, and rather broad and overplayed, as if he was still in For Make Glorious Benefit high gear, trying to amp up real-life situations to suitably absurd heights.

And, at the risk of sounding like his mum, he certainly does get carried away sometimes. Borat was perfectly pitched, still retaining the twinkle in its eye amid the Bible-belt humiliations and vengeful rodeo mobs. Trying to top that film while visibly restricted by the fact that it was getting harder for its chief assassin to mount his ambushes, Brüno ditched the charm somewhere en route. It patronised gay men, felt too complicit in the cult of celebrity it was supposed to be satirising, and fell back far too often on Baron Cohen's default weapons of scatology and cruelty. Chaplin wasn't above a little of the latter, but he never breached his essential on-screen innocence either. Baron Cohen shouldn't forget that – even if The Dictator looks like it has, on paper, that irresistible spritely irony. Otherwise, any resemblance between him and a certain comedy genius will be purely coincidental as well.